The object of the invention is to provide a method and an apparatus for controlling a multisource array emitting acoustic or seismic impulses, which makes it possible to detect possible uncontrolled triggerings.
Marine seismic prospecting methods are usually carried out by using a wave emission array and a wave reception array towed by a ship along a seismic profiling plane to be studied. The waves generated by the emission array are reflected by different reflectors of the immersed formations and are received by the reception array which generally consists of a seismic streamer of great length along which a large number of sensors are arranged.
The emission array often consists of a plurality of impulse sources towed while immersed and connected with the ship by groups of multifunction cables or umbilicals. The form of the wave produced by these impulse sources depends on the type of the sources. If the sources are of the explosion type, such as airguns for example, the main peak is produced first. With sources of the implosion type such as waterguns, the main peak is preceded by a precursory peak with a lower amplitude. Shooting detectors are integrated into the sources or arranged close to them in order to determine the triggering instants and/or the form of the produced impulses.
These sources are immersed, as the case may be, at substantially equal depths or systematically at different depths. The triggering instants are selected with precision in view of the particular layout of the emission array in the water in order to obtain a powerful and directional source. It is a matter of obtaining, by selecting the triggering instants of the different sources, the phasing of their respective main peaks in a certain direction. The operation is generally complex because multiple parameters have to be taken into account. According to its type, its depth of immersion and its mechanical state after a more or less long use, the effective instant when the main peak of the source occurs may vary within notable proportions.
A shooting synchronizer or sequencer adapted for taking into account the different parameters characterizing the emission array used is utilized to control the emission sequences of an array of immersed seismic sources.
Systems where sequencers are used for controlling impulse sources are described, for example, in U.S. Pat. Nos. 4,599,712, 4,693,336, 4,718,045, 4,739,858, and 4,757,482 and in European Patent Applications Nos. 31,196 and 48,623.
Marine seismic prospecting cycles are sometimes disrupted because some sources release themselves at an untimely moment, for various reasons generally due to leaks in the hydraulic control circuits. It may be a simple triggering delay in relation to the planned instant of a shooting sequence, or it may be new spontaneous triggering during the same seismic emission-reception cycle. In both cases, the resulting impulse produced by the emission array is disrupted, and the seismic recordings obtained are distorted.
It is therefore useful that the operator be able to supervise the running of the emission array provided that this supervision does not make the control management allotted to the shooting synchronizer used excessively complicated.